Why is light not visible when it travels.

This is a very simple question, which is why I am probably having a hard time finding the answer. Why is light not visible when it travels. You see the light of the sun, but you don't see the light travel to your eye. Light does not appear unitil is hits something, right? So the sun gives off light, the light then vanishes, travels in darkness, then appears again when it hits something or it is observed. Why?

You can only see light if it enters your eye. If light is traveling far off, how could your eye detect it? It cant. The sun gives off light, you cant see it. The light travels across space, you can see it. The light hits your eye, then you can see it.

Staff: Mentor

I don't mean to thread hijack, but this is relevant. How does a photon "carry" an image per se?

An individual photon doesn't carry an image: an image is comprised of an enormous number of individual photons. When you look around you, ever part of every object you see is sending a photon toward your eye. Millions (billions, quadrillions?) of photons of different colors hit different parts of your retina, in very much the same way that the millions individual pixels on a digital camera chip each receive millions (quadrillions?) of photons from their part of the image.

I guess that question may be more relevant in the science and study of the brain rather than physics, huh?

Nope, still physics. Optically, the human eye is not that much different from a simple digital camera (you could mount a ccd chip on the back of your eye and it would work fine if we could interface it with our brains). It's just the way the light is sensed and processed that is different.

An individual photon doesn't carry an image: an image is comprised of an enormous number of individual photons. When you look around you, ever part of every object you see is sending a photon toward your eye. Millions (billions, quadrillions?) of photons of different colors hit different parts of your retina, in very much the same way that the millions individual pixels on a digital camera chip each receive millions (quadrillions?) of photons from their part of the image. Nope, still physics. Optically, the human eye is not that much different from a simple digital camera (you could mount a ccd chip on the back of your eye and it would work fine if we could interface it with our brains). It's just the way the light is sensed and processed that is different.

That makes perfect sense. I understand lcds, I don't know why I didn't make that connection!

Thanks. I guess it goes back to light is not light until it is observed. I was using the mental image of seeing the earth and the sun from the moon. Light from the sun hits my eye, and the light reflecting on the earth will hit my eye. But I will not be able to see the light between them. So I assume what I percieve as light is only that, a perception. Light is not the same as the way we percieve it to be.

EM waves do not affect each other as they go through / past each other in space. When a light beam passes from right to left, in front of you, no energy will pass from the beam into your eye EXCEPT for the small (really really small) amount of energy that will come your way from the original source, due to diffraction at the aperture.
If you see anything of the beam it will be because of scattering by molecules and dust in front of you. What you will mainly see is the stuff in front of you.